Overlay mark design for electron beam overlay

ABSTRACT

The present disclosure provides a target and a method of performing overlay measurements on a target. The target includes an array of cells comprising a first cell, a second cell, a third cell, and a fourth cell. Each cell includes a periodic structure with a pitch. The periodic structure includes a first section and a second section, separated by a first gap. The target further includes an electron beam overlay target, such that electron beam overlay measurements, advanced imaging metrology, and/or scatterometry measurements can be performed on the target.

CROSS REFERENCE TO RELATED APPLICATIONS

This disclosure is based on, and claims priority to, U.S. ProvisionalApplication No. 63/215,561, filed on Jun. 28, 2021, the entiredisclosure of which is incorporated by reference herein.

FIELD OF THE DISCLOSURE

This disclosure relates to overlay measurements in semiconductordevices.

BACKGROUND OF THE DISCLOSURE

Evolution of the semiconductor manufacturing industry is placing greaterdemands on yield management and, in particular, on metrology andinspection systems. Critical dimensions continue to shrink, yet theindustry needs to decrease time for achieving high-yield, high-valueproduction. Minimizing the total time from detecting a yield problem tofixing it determines the return-on-investment for a semiconductormanufacturer.

Fabricating semiconductor devices, such as logic and memory devices,typically includes processing a semiconductor wafer using a large numberof fabrication processes to form various features and multiple levels ofthe semiconductor devices. For example, lithography is a semiconductorfabrication process that involves transferring a pattern from a reticleto a photoresist arranged on a semiconductor wafer. Additional examplesof semiconductor fabrication processes include, but are not limited to,chemical-mechanical polishing (CMP), etch, deposition, and ionimplantation. Multiple semiconductor devices may be fabricated in anarrangement on a single semiconductor wafer that are separated intoindividual semiconductor devices.

Current methods for optical overlay measurement rely on two maintechnologies: imaging and scatterometry. In imaging, the position ofperiodic targets is measured in the field of view of the optical systemand the overlay (OVL) is deduced from positions of targets printed indifferent layers. Scatterometry utilizes interference betweenelectromagnetic (EM) waves scattered by periodic overlay marks (targetswith periodic structures) printed at different layers to deduce therelative displacement of the layers. In both cases a control onamplitudes and phases of the diffraction orders of the scatteredelectromagnetic waves may provide a crucial effect on accuracy andprecision of overlay measurement.

When overlay errors are observed, an overlay measurement may be used toapply corrections and to keep overlay errors within desired limits. Forexample, overlay measurements may be fed into an analysis routine thatcalculates scanner corrections, as well as other statistics, which maybe used by an operator in order to better align the lithography toolused in the process. In typical fabrication operations, semiconductormanufacturing processes are run in batches called lots. A lot, or awafer lot, is defined as a quantity of wafers which are processedtogether as a single group. Conventional overlay monitor and controltechniques generally take a single set of scanner corrections and applythe same set for all wafers in the same lot.

However, improved target designs and methods of measurement are neededas semiconductor devices become more complex.

BRIEF SUMMARY OF THE DISCLOSURE

Embodiments of the present disclosure provide a target. The target maycomprise an array of cells comprising a first cell, a second cell, athird cell, and a fourth cell. Each cell may comprise a periodicstructure with a pitch. The periodic structure may include a firstsection and a second section, separated by a first gap.

According to an embodiment of the present disclosure, the target mayfurther comprise an electron beam overlay target disposed in a center ofthe array of cells comprising the first cell, the second cell, the thirdcell, and the fourth cell.

According to an embodiment of the present disclosure, the first cell,the second cell, the third cell, and the fourth cell may be arrangedhaving rotational symmetry.

According to an embodiment of the present disclosure, the periodicstructure of each cell may further include a third section separated bya second gap.

According to an embodiment of the present disclosure, the periodicstructure of the first cell may have a first cell height along the firstaxis and a first cell width along the second axis. The periodicstructure of the second cell may have a second cell height along thesecond axis and a second cell width along the first axis. The periodicstructure of the third cell may have a third cell height along thesecond axis and a third cell width along the first axis. The periodicstructure of the fourth cell may have a fourth cell height along thefirst axis and a fourth cell width along the second axis.

According to an embodiment of the present disclosure, the first cellwidth may be greater than the second cell height, the second cell widthmay be greater than the third cell height, the third cell width may begreater than the fourth cell height, and the fourth cell width may begreater than the first cell height.

According to an embodiment of the present disclosure, the first cellwidth may be equal to the second cell height, the second cell width maybe equal to the third cell height, the third cell width may be equal tothe fourth cell height, and the fourth cell width may be equal to thefirst cell height.

According to an embodiment of the present disclosure, part of theelectron beam overlay target may be a plurality of nested polygonalshapes disposed inside each other that define a center hollow area. Theinterconnected polygonal shapes may be squares.

According to an embodiment of the present disclosure, the periodicstructure of each cell may have a first height and a second heightlarger than the first height, and the periodic structure with the secondheight may be disposed adjacent the electron overlay target.

According to an embodiment of the present disclosure, the second sectionof the periodic structure of each cell may have a second height and athird height less than the second height, and the periodic structurewith the third height may be disposed adjacent the electron overlaytarget.

According to an embodiment of the present disclosure, the target mayfurther comprise a plurality of electron beam overlay targets disposedat an edge of the first cell, the second cell, the third cell, and thefourth cell. Each of the electron beam overlay targets may include abody a plurality of segments extending from the body, and a connectingsegment which connects ends of the plurality of segments.

According to an embodiment of the present disclosure, the plurality ofthe electron beam overlay targets may be disposed at the edge of thefirst cell, the second cell, the third cell, and the fourth celladjacent to the first section of the periodic structure.

According to an embodiment of the present disclosure, the plurality ofthe electron beam overlay targets may be disposed at the edge of thefirst cell, the second cell, the third cell, and the fourth celladjacent to the second section of the periodic structure.

According to an embodiment of the present disclosure, the plurality ofthe electron beam overlay targets may be disposed at the edge of thefirst cell, the second cell, the third cell, and the fourth celladjacent to the first gap.

According to an embodiment of the present disclosure, the periodicstructures may further include a second pitch between groups of aplurality of periodic structures at the pitch. The second pitch may belarger than the first pitch.

According to an embodiment of the present disclosure, the target mayfurther comprise a plurality of electron beam overlay targets. Each ofthe electron beam overlay targets may include a plurality of segmentsextending from two sides of a body. The body may be disposed betweenpairs of gratings in the first section and the second section of theperiodic structure and the plurality of segments may be disposed on thepairs of gratings in the first and second section adjacent to the body.

According to an embodiment of the present disclosure, the plurality ofelectron beam overlay targets may be disposed between pairs of gratingsat ends of the first section and the second section of the periodicstructure of each cell.

According to an embodiment of the present disclosure, the plurality ofelectron beam overlay targets may be disposed between each pair ofgratings of the first section and the second section of the periodicstructure of each cell.

Another embodiment of the present disclosure provides a method ofperforming overlay measurements on a target using a semiconductormetrology tool.

DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the disclosure,reference should be made to the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is four cell scatterometry overlay (SCOL) target with combinedplacement on an electron beam overlay (eOVL) target in center and edgeaccording to an embodiment of the present disclosure;

FIG. 2 is an advanced imaging metrology (AIM) target with combinedcenter placement on an eOVL target according to an embodiment of thepresent disclosure;

FIG. 3 is a triple AIM target with combined center placement on a tripleeOVL target according to an embodiment of the present disclosure;

FIG. 4 is an AIM target with combined placement on an eOVL target withadded segments in center and edge according to an embodiment of thepresent disclosure;

FIG. 5 is an AIM target with combined placement on an eOVL target withno added segments in center and edge according to an embodiment of thepresent disclosure;

FIG. 6 is an AIM target with combined placement on eOVL target withprolongation of segments in center and edge according to an embodimentof the present disclosure;

FIG. 7 is an AIM target with AIM polygon reduction according to anembodiment of the present disclosure;

FIG. 8 is another AIM target with AIM polygon reduction according to anembodiment of the present disclosure;

FIG. 9A is a combined AIM and eOVL target design according to anembodiment of the present disclosure;

FIG. 9B shows the eOVL target of the embodiment of FIG. 9A;

FIG. 10 is another combined AIM and eOVL target design according to anembodiment of the present disclosure;

FIG. 11 is another combined AIM and eOVL target design according to anembodiment of the present disclosure;

FIG. 12A is another combined AIM and eOVL target design according to anembodiment of the present disclosure;

FIGS. 12B and 12C show the eOVL targets of the embodiment of FIG. 12A;

FIG. 13 is another combined AIM and eOVL target design according to anembodiment of the present disclosure;

FIG. 14 is another combined AIM and eOVL target design according to anembodiment of the present disclosure;

FIG. 15 is a field layout with two types of combined AIM and eOVLtargets according to an embodiment of the present disclosure;

FIG. 16 is a field layout with single types of combined AIM and eOVLtargets according to an embodiment of the present disclosure;

FIG. 17 is another field layout with single types of combined AIM andeOVL targets according to an embodiment of the present disclosure;

FIG. 18 is a method of performing overlay measurements using asemiconductor tool according to an embodiment of the present disclosure;and

FIG. 19 is a diagram of a system according to an embodiment of thepresent disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

Although claimed subject matter will be described in terms of certainembodiments, other embodiments, including embodiments that do notprovide all of the benefits and features set forth herein, are alsowithin the scope of this disclosure. Various structural, logical,process step, and electronic changes may be made without departing fromthe scope of the disclosure. Accordingly, the scope of the disclosure isdefined only by reference to the appended claims.

In general, an overlay error between two process layers or a shiftbetween two sets of structures on the same layer may be determined usingoverlay targets, for which the target structures are designed with aknown relationship between their symmetry characteristics anddiscrepancies from such symmetry correspond to overlay error in suchtargets. As used herein, a layer may refer to any suitable materials,such as a semiconductor or a photoresist layer that are generated andpatterned for fabrication of a wafer device or test structure. Althoughthe following target examples are shown to have structures on two (ormore) layers for measuring overlay, each target may include two (ormore) sets of structures on the same layer for determining a shift errorbetween such set of structures. Additionally, embodiments of the presentdisclosure may be applied to any type of periodic targets, and suchtargets may be formed in an inactive area (e.g., scribe line) or in anactive region of a die on of a production wafer (e.g., as part of adevice portion of a die). Techniques that are described herein may alsobe applied to targets for determining other types of characteristics,such as critical dimension (CD), CD uniformity (CDU), edge placementerror (EPE), pattern fidelity, etc.

Overlay targets typically occupy an expensive location on an integratedcircuit. Semiconductor manufacturers generally seek to decreasemeasurement time and wafer area of these overlay targets. Space on awafer can be saved, measurement time can be made faster, and acomparison of results from the two technologies can be performed bycombining imaging-based and scatterometry-based targets. Thus, includingtwo targets in an area of an integrated circuit and measuring bothoverlay targets at the same time would provide benefits to thesemiconductor manufacturing process.

Embodiments disclosed herein combine electron beam overlay (eOVL) withscatterometry overlay (SCOL). Currently the scanning electron microscope(SEM) technology is used as a ruler for optical metrology measurements.Embodiments disclosed herein provide solutions with new target designsfor electron beam overlay for combination with optical measurements.Target architypes also can combine optical image-based overlay (IBO) anddiffraction-based overlay (DBO) targets with electron beam overlayspecialized targets. Alternating the combined targets can enable densersampling to increase model accuracy because the number of measurementsamples on the wafer increases while keeping the wafer real estateidentical. The mixed targets use 100% real estate available for overlaymetrology, which provides a 2× increased number of targets for bothtechnologies.

Target designs disclosed herein can reduce measurementmove-acquire-measurement (MAM) time and improve measurements accuracybecause the regions of interest (ROI) can contain both X and Yinformation content within small field of view (FOV). For example, inone ROI, one grating may be for information in the X direction, andanother grating may be for information in the Y direction. It may not benecessary to move to two different targets because all the informationis contained in one target. Thus, the disclosed targets increase theinformation content per area.

The measurements can be applied on coarse and fine pitch of thedifferent target architypes. The target stack materials can vary betweenline and space features. The reverse combination use case can use thecurrent optical targets with a denser sampling of electron beam overlaytargets. A combined design of four cell scatterometry overlay andelectron beam overlay also can be used.

The embodiments disclosed herein can enable scatterometry target axisseparation, which contains two cells per direction as in standard fourcell design with prolonged bars in both directions of each layer percell. This can provide physical separation between the differentalignment scheme layers. The region of interest placement on such targettype can be separated for each layer. Since multiple region of interestused per direction targets architecture of bar over bar and bar overtrench with induced overlay. This can be used for scatterometry target(f0) measurable regardless of their shift coverage. Both directions canbe applicable within the same target as well with prolonged bars peraxis.

Any design image-based overlay or diffraction-based overlay can enablean electron beam overlay target, center of electron beam overlay, andoptical target that coincide. This can eliminate process variationeffect, placement error effect, and can improve the ability to correctof the modeled terms. Due to design rules, the electron beam overlaytarget may be placed as close as possible to the optical target.

Additional segments can be added to the optical target to fill in thespace occupied by electron beam overlay target placement or prolongationof the bars, which can enable more information content.

FIG. 1 is a four cell scatterometry overlay (SCOL) target with combinedplacement on electron beam overlay (eOVL) target in center and edge. Forexample, scatterometry uses the areas where layers of gratings overlap,while electron beam overlay uses the areas where layers of gratings areseparated. The target 100 includes a first periodic structure 101 and asecond periodic structure 102. These are shown with different shadingfor ease of illustration. The first periodic structure 101 is formed ona first layer of a semiconductor device and has a first pitch along anaxis and the second periodic structure 102 is formed on a second layerof the semiconductor device and has a second pitch along that axis,different from the first pitch. When suitably illuminated, the first andsecond periodic structures 101, 102 together form a pattern. The layerscan be adjacent to each other or can be separated by other layers. Thefirst periodic structure 101 and second periodic structure 102 areillustrated as see-through for ease of illustration.

Each of a plurality of gratings in the first periodic structure 101 hasa first width along a second axis (e.g., X) and a first height along afirst axis (e.g., Y). The first width may be between 1 μm and 100 μm.The first height may be between 10 μm and 100 μm. The first width andthe first height may be larger or smaller than these ranges depending onthe application. The first periodic structure 101 has a first pitch. Thefirst pitch may be between 200 nm and 1000 nm. The first pitch may belarger or smaller than this range depending on the application.

A second periodic structure 102 is formed on a second layer of thesemiconductor device. Each of a plurality of gratings in the secondperiodic structure 102 has a second width along the second axis (e.g.,X) and a second height along the first axis (e.g., Y). The second widthof the second periodic structure 102 is narrower than the first width ofthe first periodic structure 101. The second width may be between 1 μmand 100 μm. The second height may be between 10 μm and 100 μm. Thesecond width and the second height may be larger or smaller than theseranges depending on the application. The second periodic structure 102has a second pitch less than the first pitch of the first periodicstructure 101. The second pitch may be between 200 nm and 1000 nm. Thesecond pitch may be larger or smaller than this range depending on theapplication. The second pitch may be larger than the first pitch. Thegratings of the first periodic structure 101 are positioned over thegratings of the second periodic structure 102.

The first and second periodic structures 101, 102 are mutually overlaidand are formed of arrays of mutually parallel lines on the first andsecond layers of the semiconductor device. The lines are arranged inregistration along an axis, such as the Y-axis. The structure sizes,dimensions, and angles of the fine segmentation may depend on thesemiconductor manufacturer, technology node, and processing step. Nodesmay operate at structure sizes of a few nm to several μm. In the case ofdiagonal structures, 15° to 30° may be common, but other angles arepossible.

Target 100 may be 10 μm to 60 μm in side length. For example, the sidelengths of the target 100 may be not greater than 40 μm. In general, thesize of the target 100 may depend on the selected optical wavelength andbeam size. Larger targets 100 may make measurements easier, but may alsotake up more area on the semiconductor wafer. Reducing the size of thetarget 100 may reduce cost of production and increase available space onthe wafer. The particular balance of physics and costs to select atarget size may depend on the design rules of the process of thespecific layer and/or product. The target 100 can be rectangular inshape.

The target 100 further includes an eOVL structure 103. The eOVLstructure 103 may be disposed in a center of the target 100. The eOVLstructure 103 may be polygonal. For example, the eOVL structure may berectangular.

FIG. 2 is an advanced imaging metrology (AIM) target with combinedcenter placement on eOVL target. The target 200 may comprise an array ofcells. Each cell may include a periodic structure with a pitch. Forexample, the periodic structure may comprise a plurality of rectangulargratings, where the pitch refers to the distance between adjacentgratings. The periodic structure may include a first section 201 and asecond section 202, which are separated by a first gap 204. Target 200differs from target 100 in that the periodic structures of the firstsection 201 and the second section 202 are separated, while the firstperiodic structure 101 and the second periodic structure 102 overlap. Ingeneral, the size of the periodic structures of the target 200 may besimilar to those of the target 100 described above.

The array of cells may comprise a first cell 200A, a second cell 200B, athird cell 200C, and a fourth cell 200D. The second cell 200B may bearranged along a first axis X and rotated 90 degrees relative to thefirst cell 200A. The third cell 200C may be arranged along a second axisY and rotated −90 degrees relative to the first cell 200A. The fourthcell 200D may be arranged along the first axis X relative to the thirdcell 200C, arranged along the second axis Y relative to the second cell200B, and rotated 180 degrees relative to the first cell 200A.Accordingly, the array of cells may be arranged in a 2×2 grid, havingrotational symmetry. Other arrangements of cells are possible, including3×3, 4×4, or larger grids.

The target 200 may further comprise an electron beam overlay target 210disposed in a center of the array of the first cell 200A, the secondcell 200B, the third cell 200C, and the fourth cell 200D. The electronbeam overlay target 210 may be disposed in other locations of the target200 depending on the application. The electron beam overlay target 210may be about 2 μm in size, but smaller or larger sizes are possible.

The periodic structure of the first cell 200A may have a first cellheight along the first axis X and a first cell width along the secondaxis Y. The periodic structure of the second cell 200B may have a secondcell height along the second axis Y and a second cell width along thefirst axis X. The periodic structure of the third cell 200C may have athird cell height along the second axis Y and a third cell width alongthe first axis X. The periodic structure of the fourth cell 200D mayhave a fourth cell height along the first axis X and a fourth cell widthalong the second axis Y.

Target 200 may be 20 μm by 20 μm in size, and may contain a total of 80lines (20 lines per layer and direction and two layers). The individuallines may be 1 μm in size and 2.5 μm apart. Other sizes of the target200 and the individual lines and spacings are possible.

FIG. 3 is a triple AIM target with combined center placement on tripleeOVL target. The target 300 differs from the target 200 in that theperiodic structure of each cell may further include a third section 203.The second section 202 and the third section 203 may be separated by asecond gap 205. The target 300 may be larger than target 200 toaccommodate placement of the third section 203, but the target 300 andits structures may be similar in size to those of the target 200described above.

FIG. 4 is an AIM target with combined placement on eOVL target withadded segments in center and edge. The target 400 differs from thetarget 200 in that the periodic structure of each cell may includeadditional gratings. The first section 201 and the second section 202 ofeach cell may each include additional gratings added at the edge of thetarget 400 and/or at the center of the target 400. The presence ofadditional gratings in the target 400 may improve measurementperformance as the signal strength increases. The size of the target 400and its structures may be similar in size to those of target 200described above.

According to an embodiment of the present disclosure, the first cellwidth may be greater than the second cell height. The second cell widthmay be greater than the third cell height. The third cell width may begreater than the fourth cell height. The fourth cell width may begreater than the first cell height. For example, as shown in FIG. 4 ,each cell in the array of the target 400 may be rectangular. In thisarrangement, measurement performance may be improved without sacrificingreal estate on the wafer.

FIG. 5 is an AIM target with combined placement on eOVL target with noadded segments in center and edge. The target 500 differs from thetarget 200 in that the periodic structure of each cell may include fewergratings. The first section 201 and the second section 202 of each cellmay each have fewer gratings at the edge of the target 500 and/or at thecenter of the target 500 than the target 200. The presence of fewergratings in the target 500 may reduce production cost and wafer spaceused by the target 500. The size of the target 500 and its structuresmay be similar in size to those of target 200 described above.

According to an embodiment of the present disclosure, the first cellwidth may be equal to the second cell height. The second cell width maybe equal to the third cell height. The third cell width may be equal tothe fourth cell height. The fourth cell width may be equal to the firstcell height. For example, as shown in FIG. 5 , each cell in the array ofthe target 500 may be square. In this arrangement, measurementperformance may be improved without sacrificing real estate on thewafer.

FIG. 6 is an AIM target with combined placement on eOVL target withprolongation of segments in center and edge. The target 600 differs fromthe target 200 in that the periodic structure of each cell may includeprolonged gratings. For example, the first periodic structure 201 mayhave a first height and the second periodic structure 202 may have asecond height. The second height may be greater than the first height.In this way, the second periodic structure 202 of each cell may extendalong each side of the centrally-located eOVL target 210. The ratio ofthe first height and the second height may depend on the design rules ofthe particular application. The size of the target 600 and itsstructures may be similar in size to those of target 200 describedabove.

FIG. 7 is an AIM target with AIM polygon reduction for optimization ofreal estate, process robustness, and dummification. As shown in thetarget 700, the eOVL target 210 may be a plurality of nested polygonalshapes disposed inside each other that define a center hollow area. Theinterconnected polygonal shapes may be squares, but other shapes may bepossible. Dummification structures may be used to fill empty areas(e.g., white areas in FIG. 7 ) of the target 700 (or of other targets ofthe present disclosure) or to gradually change between large and smallstructures without interfering with target design.

The target 700 may also differ from the target 200 in that the periodicstructure of each cell may include shorter gratings 202 a. For example,the second periodic structure 202 may include gratings with a secondheight and a third height. The third height may be less than the secondheight. The ratio of the second height and the third height may dependon the design rules of the particular application. The shorter gratings202 a having the third height may be disposed adjacent to the eOVLtarget 210. In this way, the size of the eOVL target 210 may beincreased, fitting within the space of the vacated by the shortergratings 202 a, which optimizes the real estate of the target 700. Thesize of the target 700 and its structures may be similar in size tothose of target 200 described above.

FIG. 8 is another AIM target with AIM polygon reduction for optimizationof real estate, process robustness, and dummification. The target 800differs from the target 700 in that the second periodic structure ofeach cell may include multiple shorter gratings 202 a having the thirdheight. In this way, the size of the eOVL target 210 may be furtherincreased, fitting within the increased space vacated by the shortergratings 202 a, which further optimizes the real estate of the target800. The size of the target 800 and its structures may be similar insize to those of target 200 described above.

FIG. 9A is a combined AIM and eOVL target design. As shown in FIG. 9B,the eOVL target 210 of the target 900 may include a body 211 and aplurality of segments 212 extending from the body 211. The plurality ofsegments 212 may be connected to each other by a connecting segment 213at the ends extended away from the body 211. In this arrangement, theeOVL target 210 may define a comb-like structure. Similar structures maybe used for electrical tests on a wafer. Combining existing designs ofother types of metrology with optical overlay targets can be beneficial,as it can reduce the total number required for a particular design.

The target 900 may include a plurality of eOVL targets 210. For example,each cell may include two eOVL targets 210 arranged adjacent to thesecond periodic structure 202 at opposite ends of each cell. Each eOVLtarget 210 may be arranged adjacent to the shorter gratings 202 a of thesecond periodic structure 202. For example, each eOVL target 210 may bearranged perpendicular to the shorter gratings 202 a of each cell. EacheOVL target 210 may be arranged on the same side of the second periodicstructure 202 of each cell. For example, each eOVL target 210 may bearranged opposite to the first gap 204 of each cell. The shortergratings 202 a may be located in the second periodic structure 202toward the center of the target 900 and toward the edges of the target900. In this arrangement, the target 900 may have rotational symmetrybased on the placement of the eOVL targets 210 among the first periodicstructures 201 and the second periodic structures 202.

According to an embodiment of the present disclosure, the periodicstructures of each cell may include a first pitch and a second pitch.For example, individual gratings within the first periodic structures201 and the second periodic structures 202 may be separated by a firstpitch, and groups of periodic structures may be separated by the secondpitch. The second pitch may be larger than the first pitch. The ratio ofthe first pitch and the second pitch may depend on the design rules ofthe particular application. The second periodic structure 202 of eachcell may include a group of the shorter gratings 202 a. The group ofshorter gratings 202 a may be separated from the other gratings in thesecond periodic structure 202 by the second pitch. The size of thetarget 900 and its structures may be similar in size to those of target200 described above.

FIG. 10 is another combined AIM and eOVL target design. The target 1000differs from the target 900 in that each cell may include two eOVLtargets 210 arranged adjacent to the first periodic structure 201 atopposite ends of each cell. The first periodic structure 201 may includeshorter gratings 201 a having the third height, shorter than the firstheight. Each eOVL target 210 may be arranged perpendicular to theshorter gratings 201 a of each cell. Each eOVL target 210 may bearranged on the same side of the first periodic structure 201 of eachcell. For example, each eOVL target 201 may be arranged opposite to thefirst gap 204 of each cell. The shorter gratings 201 a may be located inthe first periodic structure 201 at opposite ends of each cell, on theouter edges of the target 1000. In this arrangement, the target 1000 mayhave rotational symmetry based on the placement of the eOVL targets 210among the first periodic structures 201 and the second periodicstructures 202. The size of the target 1000 and its structures may besimilar in size to those of target 200 described above.

FIG. 11 is another combined AIM and eOVL target design. The target 1100differs from the target 900 in that the eOVL targets 210 may be arrangedon opposite sides of the second periodic structure 202 of each cell. Forexample, one of the eOVL targets 210 may be arranged adjacent to thefirst gap 204, and the other of the eOVL targets 210 may be arrangedopposite to the first gap 204 of each cell. In this arrangement, thetarget 1100 may have rotational symmetry based on the placement of theeOVL targets 210 among the first periodic structures 201 and the secondperiodic structures 202. The size of the target 1100 and its structuresmay be similar in size to those of target 200 described above.

FIG. 12A is another combined AIM and eOVL target design. As shown inFIG. 12B and FIG. 12C, the eOVL target 1210 of the target 1200 mayinclude a body 1211 and a plurality of segments 1212 extending from bothsides the body 1211. The body 1211 may be disposed between two gratingsof the first section 1201 (FIG. 12B) and the second section 1202 (FIG.12C) of the periodic structure of each cell, and the plurality ofsegments may be disposed on the first section 1201 and the secondsection 1202 respectively. The body 1211 may be comparable to existingAIM targets, which may be beneficial in certain cases. The two gratingsof the first section 1201 and the second section 1202 disposed on eitherside of the body 1211 may have a width and a pitch that is less thanthat of other gratings in the first section 1201 and the second section1202. The first section 1201 and the second section 1202 of the periodicstructure of each cell may be separated by a first gap 1204. In thisarrangement, the eOVL target 210 may be mixed optical target whilekeeping the overall symmetry. This may be beneficial where havingdifferent pitches close by is problematic for manufacturing, wheredummification structures may be otherwise placed.

The target 1200 may include a plurality of eOVL targets 1210. Forexample, each cell may include a plurality of eOVL targets 1210 disposedalternately between the first section 1201 and the second section 1202of the periodic structure. As shown in FIG. 12A, the target 1200 mayinclude two eOVL targets 1210 arranged between the gratings of the firstsection 1201 and three eOVL targets 1210 arranged on either side of eachof the gratings of the second section 1202. The inversion of the layercombinations between FIG. 12B and FIG. 12C may be beneficial as it mayensure that the signal content between the layers is as similar aspossible, such that no layer signal will dominate the overlay result.The size of the target 1200 and its structures may be similar in size tothose of the target 200 described above.

FIG. 13 is another combined AIM and eOVL target design. The target 1300differs from the target 1200 in that the plurality of eOVL targets 1210disposed in each cell are disposed at the ends of the first section 1201and the second section 1202 of the periodic structure. In this way, theplurality of eOVL targets 1210 may be disposed adjacent to the center ofthe target 1300 and/or adjacent to the outer edges of the target 1300.This symmetry of the placement of the eOVL targets 1210 in the firstsection 1201 and the second section 1202 may be beneficial under certaindesign rules. The size of the target 1300 and its structures may besimilar in size to those of the target 200 described above.

FIG. 14 is another combined AIM and eOVL target design. The target 1400differs from the target 1200 in that the plurality of eOVL targets 1210are disposed between each pair of gratings of the first section 1201 andthe second section 1202 of the periodic structure of each cell. In thisway, each cell of the target 1400 may comprise the plurality of eOVLtargets 1210 arranged consecutively within the first section 1201 andthe second section 1202 of the periodic structure. The size of thetarget 1400 and its structures may be similar in size to those of thetarget 200 described above. In this arrangement, the target 1400 may befully regular and symmetric, which may be beneficial under some designrules.

FIG. 15 is a field layout with two types of combined AIM and eOVLtargets. The layout 1500 may comprise a central target 1510 and aplurality of outer targets 1520 arranged around the periphery of thecentral target 1510. The size of the layout 1500 and the spacing betweenthe targets may depend on the design rules of the particularapplication. The central target 1510 may be larger than the plurality ofouter targets 1520. The ratio of the size of the central target 1510 tothe outer targets 1520 may be defined by balancing the relative signalstrengths and the available area on the wafer. The central target 1510and the plurality of outer targets 1520 may be any of the target designsdescribed herein. The plurality of outer targets 1520 may include afirst target type 1521 and a second target type 1522. The first targettype 1521 and the second target type 1522 may be any two differenttarget designs described herein or combination thereof. One of the firsttarget type 1521 and the second target type 1522 may be of the sametarget design as the central target 1510. Alternatively, the firsttarget type 1521 and the second target type 1522 may be different targetdesigns from the central target 1510. The plurality of outer targets1520 may be arranged around the periphery of the central target 1510 inan alternating manner. For example, as shown in FIG. 15 , the pluralityof outer targets 1520 may alternate between the first target type 1521and the second target type 1522. By alternating target types, accuracyper target may be improved, and target noise may be reduced. As eachtarget design can have its own accuracy errors and offsets, it may bepossible to choose targets for the layout 1500 in such a way that theaccuracy errors and offsets may at least partially cancel out, and maythereby improve accuracy.

FIG. 16 is a field layout with single types of combined AIM and eOVLtargets. The layout 1600 differs from the layout 1500 in that theplurality of outer targets 1520 includes only the first target type1521. The first target type 1521 may be different from the target designof the central target 1510. The size of the layout 1600 and the spacingbetween the targets may depend on the design rules of the particularapplication.

FIG. 17 is another field layout with single types of combined AIM andeOVL targets. The layout 1700 differs from the layout 1500 in that theplurality of outer targets 1520 includes only the second target type1522. The second target type 1522 may be the same as the target designof the central target 1510. The size of the layout 1700 and the spacingbetween the targets may depend on the design rules of the particularapplication.

FIG. 18 is a flowchart of a method 1800. In the method, a target isprovided at 1801. The target can be one of the targets disclosed herein.Overlay measurements are performed at 1802 using a semiconductormetrology tool. The semiconductor metrology tool may be a conventionaltool, which utilizes conventional processing algorithms, or may be anelectron beam tool, which uses different measurement techniques. Forexample, one beam spot can be applied to a grating. A singlereflectivity value is collected per grating. Each cell will contain aninduced offset to calculate overlay. At least four cells with differentinduced offsets may be used. This method can be propagated until “n”cells occur. A beam spot can be scanned to average spot positionaccuracy.

FIG. 19 is a diagram of a system 1900. The system 1900 includes a chuck1902 configured to hold a wafer 1901. The wafer 1901 includes one ormore overlay targets, such as those described in the embodiments herein.

The system 1900 includes an imaging optical system 1903 configured tomeasure the overlay target on the chuck 1902. The imaging optical system1903 can include an illumination source oriented to direct radiationonto a specific location of the wafer 1901 and one or more detectorsoriented to detect an optical signal which from the wafer 1901. Theillumination source in the optical system 1903 can generate anillumination beam directed at the wafer 1901. The imaging optical system1903 also can include various lenses, optical components, other steeringdevices, or other beam transport devices. The illumination beam can bean optical beam or an electron beam.

The imaging optical system 1903 can be used for acquisition such thatthe imaging optical system 1903 is part of the acquisition sequence ofthe scatterometry measurement.

The system 1900 includes a scatterometry system 1904 configured tomeasure the overlay target on the chuck 1902. The scatterometry system1904 can be configured to measure the same overlay target as the imagingoptical system 1903.

The scatterometry system 1904 can include an illumination sourceoriented to direct radiation onto a specified location of the wafer 1901and one or more detectors oriented to detect a scatterometry signalwhich has been scattered by the wafer 1901. The illumination source inthe scatterometry system 1904 can generate an illumination beam directedat the wafer 1901. The scatterometry system 1904 also can includevarious lenses or optical components. The scatterometry system 1904 canuse the imaging optical system 1903 or information from the imagingoptical system 1903 during the scatterometry sequence.

Measurements of the wafer 1901 by the imaging optical system 1903 andthe scatterometry system 1904 can be performed while the wafer 1901remains on the chuck 1902. Thus, the wafer 1901 may not move betweenmeasurement by the imaging optical system 1903 and the scatterometrysystem 1904. In an instance, a vacuum around the wafer 1901 is notbroken between measurements by the imaging optical system 1903 and thescatterometry system 1904. In an instance, one of the measurements bythe imaging optical system 1903 and the scatterometry system 1904 occursafter the other. In another instance, the measurements by the imagingoptical system 1903 and the scatterometry system 1904 occur at leastpartially simultaneously or at the same time.

While disclosed as one system 1900, the imaging optical system 1903 andthe scatterometry system 1904 can be in two separate systems. Each ofthe systems may have a separate chuck.

A processor 1905 is in electronic communication with an electronic datastorage unit 1906, the imaging optical system 1903, and thescatterometry system 1904. The processor 1905 may include amicroprocessor, a microcontroller, or other devices. The processor 1905can receive output from the imaging optical system 1903 and thescatterometry system 1904.

The system 1900 can include an imaging optical system acquisition module1907 configured to acquire the overlay target on the chuck 1902 with theimaging optical system 1903. The optical system acquisition module 1907forms an acquisition image that can be used by the imaging opticalsystem 1903 or the scatterometry system 1904. While illustrated asseparate units, the imaging optical system acquisition module 1907 maybe part of the imaging optical system 1903. The imaging optical systemacquisition module 1907 also may be part of the processor 1905.

The processor 1905 and electronic data storage unit 1906 may be part ofthe system 1900 or another device. In an example, the processor 1905 andelectronic data storage unit 1906 may be part of a standalone controlunit or in a centralized quality control unit. Multiple processors 1905or electronic data storage unit 1906 may be used. In an embodiment, theprocessor 1905 may be disposed in the system 1900.

The processor 1905 may be implemented in practice by any combination ofhardware, software, and firmware. Also, its functions as describedherein may be performed by one unit, or divided up among differentcomponents, each of which may be implemented in turn by any combinationof hardware, software and firmware. Program code or instructions for theprocessor 1905 to implement various methods and functions may be storedin readable storage media, such as a memory in the electronic datastorage unit 1906 or other memory.

The processor 1905 may be coupled to the components of the system 1900in any suitable manner (e.g., via one or more transmission media, whichmay include wired and/or wireless transmission media) such that theprocessor 1905 can receive output. The processor 1905 may be configuredto perform a number of functions using the output.

The processor 1905, other system(s), or other subsystem(s) describedherein may be part of various systems, including a personal computersystem, image computer, mainframe computer system, workstation, networkappliance, interne appliance, or other device. The subsystem(s) orsystem(s) may also include any suitable processor known in the art, suchas a parallel processor. In addition, the subsystem(s) or system(s) mayinclude a platform with high speed processing and software, either as astandalone or a networked tool.

If the system includes more than one subsystem, then the differentsubsystems may be coupled to each other such that images, data,information, instructions, etc. can be sent between the subsystems. Forexample, one subsystem may be coupled to additional subsystem(s) by anysuitable transmission media, which may include any suitable wired and/orwireless transmission media known in the art. Two or more of suchsubsystems may also be effectively coupled by a shared computer-readablestorage medium (not shown).

The processor 1905 may be configured to perform a number of functionsusing the output of the imaging optical system 1903, and thescatterometry system 1904, or other output. For instance, the processor1905 may be configured to send measurement results of the wafer 1901. Inanother example, the processor 1905 can determine electron beam overlayand/or scatterometry overlay error within the target. In anotherexample, the processor 1905 may be configured to send the output to anelectronic data storage unit 1906 or another storage medium. Theprocessor 1905 may be further configured as described herein.

The processor 1905 may be configured according to any of the embodimentsdescribed herein. The processor 1905 also may be configured to performother functions or additional steps using the output of the system 1900or using images or data from other sources.

In another embodiment, the processor 1905 may be communicatively coupledto any of the various components or sub-systems of system 1900 in anymanner known in the art. Moreover, the processor 1905 may be configuredto receive and/or acquire data or information from other systems (e.g.,inspection results from an inspection system such as a review tool, aremote database including design data and the like) by a transmissionmedium that may include wired and/or wireless portions. In this manner,the transmission medium may serve as a data link between the processor1905 and other subsystems of the system 1900 or systems external tosystem 1900.

In some embodiments, various steps, functions, and/or operations ofsystem 1900 and the methods disclosed herein are carried out by one ormore of the following: electronic circuits, logic gates, multiplexers,programmable logic devices, ASICs, analog or digital controls/switches,microcontrollers, or computing systems. Program instructionsimplementing methods such as those described herein may be transmittedover or stored on carrier medium. The carrier medium may include astorage medium such as a read-only memory, a random access memory, amagnetic or optical disk, a non-volatile memory, a solid state memory, amagnetic tape, and the like. A carrier medium may include a transmissionmedium such as a wire, cable, or wireless transmission link.

For instance, the various steps described throughout the presentdisclosure may be carried out by a single processor 1905 (or computersystem) or, alternatively, multiple processors 1905 (or multiplecomputer systems). Moreover, different sub-systems of the system 1900may include one or more computing or logic systems. Therefore, the abovedescription should not be interpreted as a limitation on the presentdisclosure but merely an illustration.

An additional embodiment relates to a non-transitory computer-readablemedium storing program instructions executable on a processor forinspecting a wafer, as disclosed herein. In particular, a processor,such as the processor 1905, can be coupled to a memory in an electronicdata storage medium, such as the electronic data storage unit 1906, withnon-transitory computer-readable medium that includes executable programinstructions. The computer-implemented method may include any step(s) ofany method(s) described herein. For example, processor 1905 may beprogrammed to perform some or all of the steps of FIG. 18 . The memoryin the electronic data storage unit 1906 may be a storage medium such asa magnetic or optical disk, a magnetic tape, or any other suitablenon-transitory computer-readable medium known in the art.

The program instructions may be implemented in any of various ways,including procedure-based techniques, component-based techniques, and/orobject-oriented techniques, among others. For example, the programinstructions may be implemented using ActiveX controls, C++ objects,JavaBeans, Microsoft Foundation Classes (MFC), Streaming SIMD Extension(SSE), or other technologies or methodologies, as desired.

In another embodiment, a method of designing of the targets disclosedherein can be used. The method may be performed with a processor, suchas those described therein. The gratings of the first and secondperiodic structures of the targets disclosed herein can be optimized.

While specific configurations are shown in the embodiments illustratedherein, variations that include all or some of the features of theseembodiments are possible.

Although the present disclosure has been described with respect to oneor more particular embodiments, it will be understood that otherembodiments of the present disclosure may be made without departing fromthe scope of the present disclosure. Hence, the present disclosure isdeemed limited only by the appended claims and the reasonableinterpretation thereof.

What is claimed is:
 1. A target comprising: an array of cells comprisinga first cell, a second cell, a third cell, and a fourth cell, eachcomprising a periodic structure with a pitch, wherein the periodicstructure includes a first section and a second section, separated by afirst gap; and an electron beam overlay target disposed in a center ofthe array of cells comprising the first cell, the second cell, the thirdcell, and the fourth cell.
 2. The target of claim 1, wherein the firstcell, the second cell, the third cell, and the fourth cell are arrangedhaving rotational symmetry.
 3. The target of claim 1, wherein theperiodic structure of each cell further includes a third sectionseparated by a second gap.
 4. The target of claim 1, wherein theperiodic structure of the first cell has a first cell height along thefirst axis and a first cell width along the second axis, the periodicstructure of the second cell has a second cell height along the secondaxis and a second cell width along the first axis, the periodicstructure of the third cell has a third cell height along the secondaxis and a third cell width along the first axis, and the periodicstructure of the fourth cell has a fourth cell height along the firstaxis and a fourth cell width along the second axis, and wherein thefirst cell width is greater than the second cell height, the second cellwidth is greater than the third cell height, the third cell width isgreater than the fourth cell height, and the fourth cell width isgreater than the first cell height.
 5. The target of claim 1, whereinthe periodic structure of the first cell has a first cell height alongthe first axis and a first cell width along the second axis, theperiodic structure of the second cell has a second cell height along thesecond axis and a second cell width along the first axis, the periodicstructure of the third cell has a third cell height along the secondaxis and a third cell width along the first axis, and the periodicstructure of the fourth cell has a fourth cell height along the firstaxis and a fourth cell width along the second axis, and wherein thefirst cell width is equal to the second cell height, the second cellwidth is equal to the third cell height, the third cell width is equalto the fourth cell height, and the fourth cell width is equal to thefirst cell height.
 6. The target of claim 1, wherein part of theelectron beam overlay target is a plurality of nested polygonal shapesdisposed inside each other that define a center hollow area.
 7. Thetarget of claim 6, wherein the interconnected polygonal shapes aresquares.
 8. The target of claim 6, wherein the periodic structure ofeach cell have a first height and a second height larger than the firstheight, and wherein the periodic structure with the second height isdisposed adjacent the electron overlay target.
 9. The target of claim 6,wherein the second section of the periodic structure of each cell has asecond height and a third height less than the second height, andwherein the periodic structure with the third height is disposedadjacent the electron overlay target.
 10. A method of performing overlaymeasurements on the target of claim 1 using a semiconductor metrologytool.
 11. A target comprising: an array of cells comprising a firstcell, a second cell, a third cell, and a fourth cell, each comprising aperiodic structure with a pitch, wherein the periodic structure includesa first section and a second section, separated by a first gap; and aplurality of electron beam overlay targets disposed at an edge of thefirst cell, the second cell, the third cell, and the fourth cell,wherein each of the electron beam overlay targets includes a body, aplurality of segments extending from the body, and a connecting segmentwhich connects ends of the plurality of segments.
 12. The target ofclaim 11, wherein the plurality of the electron beam overlay targets aredisposed at the edge of the first cell, the second cell, the third cell,and the fourth cell adjacent to the first section of the periodicstructure.
 13. The target of claim 11, wherein the plurality of theelectron beam overlay targets are disposed at the edge of the firstcell, the second cell, the third cell, and the fourth cell adjacent tothe second section of the periodic structure.
 14. The target of claim13, wherein the plurality of the electron beam overlay targets aredisposed at the edge of the first cell, the second cell, the third cell,and the fourth cell adjacent to the first gap.
 15. The target of claim11, wherein the periodic structures further includes a second pitchbetween groups of a plurality of periodic structures at the pitch,wherein the second pitch is larger than the first pitch.
 16. A method ofperforming overlay measurements on the target of claim 11 using asemiconductor metrology tool.
 17. A target comprising: an array of cellscomprising a first cell, a second cell, a third cell, and a fourth cell,each comprising a periodic structure with a pitch, wherein the periodicstructure includes a first section and a second section, separated by afirst gap; and a plurality of electron beam overlay targets, whereineach of the electron beam overlay targets includes a plurality ofsegments extending from two sides of a body, wherein the body isdisposed between pairs gratings in the first section and the secondsection of the periodic structure and the plurality of segments aredisposed on the pairs of gratings in the first and second sectionadjacent to the body.
 18. The target of claim 17, wherein the pluralityof electron beam overlay targets are disposed between pairs of gratingsat ends of the first section and the second section of the periodicstructure of each cell.
 19. The target of claim 17, wherein theplurality of electron beam overlay targets are disposed between eachpair of gratings of the first section and the second section of theperiodic structure of each cell.
 20. A method of performing overlaymeasurements on the target of claim 16 using a semiconductor metrologytool.